The quality of air in houses and buildings is subject in a recently released Environmental Protection Agency Report titled "EPA Report to Congress on Indoor Air Quality", released Aug. 4, 1989. In that report reference is made to the so called "Sick Building Syndrom" and a program of increased research and information dissemination regarding the dangers of poor indoor air quality are recommended. Health effects attributed to poorly ventilated houses and buildings range from eye, ear, nose and throat irritation, to full scale respiratory and neurological diseases, genetic mutations and cancer. Contaminates such as radon, asbestos, tobacco smoke, formaldehyde, volatile organic compounds, chlorinated solvants, biological contaminants and pesticides etc., and the synergistic effects of multiple contaminants are cited as causes of health problems.
The report suggests that controlling the sources of contaminants is the most direct and dependable option in overcomming the problem, and that while air cleaning equipment can compliment air quality improvement, there is no substitute for providing an adequate ventilating outside air volume flow rate through a house or building.
In recent years, the high cost of energy has led many people to strive to make their houses and buildings more tightly sealed, hence, in combination with the use of insulation, more energy efficient. Said efforts have included sealing cracks and other openings in their houses or buildings to prevent heated or cooled air from escaping, and outside air which requires heating or cooling, from randomly entering at an excessive rate. In effect, such houses and buildings become, to various degrees, closed systems. In such structures the inside air turn over rate is often reduced to far below the American Association of Heating, Refrigeration and Air Conditioning Engineers presently recommended outside air volume flow rate of 15 Cubic Feet per Minute (CFM) per inhabitant, or 35% air change per hour, whichever is greater, (see ASHRAE Standard 62-1989). The result of an insufficient air volume flow rate into, and out of, such tight houses or buildings is that contaminates accumulate inside same to dangerous health affecting levels. The savings in energy costs become less significant, and might be expected to be surpassed, over time, by increased health care costs. To emphasise this point, it is estimated by some health care researchers that presently two non-smoking persons per hour, in the United States alone, contract lung cancer as a result of contact with radon in poorly ventilated houses and buildings.
Given that, aside from the potential health hazards, making houses and buildings more energy efficient is desirable, then it follows that a method which would provide a sufficient health maintaining ventilation air volume flow rate to a tight structure would be of great benefit. A search of existing Patents shows that numerous inventors have realized this and have proposed systems, and methods of their use, which provide controlled ventilation to houses and buildings. The various approaches basically utilize a source of air flow, such as a motor driven blower, to cause air to move into and out of a house or building. The air volume flow rate is typically, but not necessarily in the most basic schemes, controlled based upon signals developed by sensing air pressure differences between the inside and the outside of a house or building, from signals derived from sensed rates of air flows in various parts of a system, or by sensing the velocity of the wind outside the house or building.
The most basic schemes simply provide a large air volume flow rate into a house or building sufficient to raise air the pressure inside the house or building to a large positive value with respect to that outside the house or building. In such a scheme the air volume flow rate must be large enough to maintain the large positive pressure difference no matter what active or passive exhaust air flows develop. As an example, operating a cloths dryer or fireplace will actively force exhaust air from a house, and opening a door to the outside of the house or building can passively increase exhaust air. The problem with such simple large positive pressure systems is that they are wasteful of energy. The large volume of air which is flowed into a house or building equipped with such a system must be heated or cooled continuously. As very large air volume flow rates are not necessary to keep contaminant concentration levels low enough for health maintenance reasons, there is no valid reason to provide them to a tight house or building. Inventors have noted this and responded. For instance, Lorenz, in U.S. Pat. No. 3,611,906 and Van Huis in U.S. Pat. No. 4,043,256 teach systems which sense inside and outside air and from same develop signals which are used to control the amount of air volume flow rate through a house or building, based upon the difference in said signals. That is, the flow of air into and out of a house or building is modified as required, by use of inlet air or exhaust air fans, to dynamically keep the inside air pressure above that outside the house or building. The problem with such schemes is that outside air pressure is used as a reference, and because of quickly occurring wind induced changes, that reference is not particularly constant. A Russian Patent to Slavin et al., No. SU-590-556 teaches a system which goes some distance toward overcoming this defect by sensing wind velocity and combining a wind volocity derived signal with an outside atmospheric pressure derived signal, which combined signal is used as a basis to control air volume flow rates. The problem still remaining is, however, that the wind can change very quickly and control systems tend to become unstable when a reference signal changes quickly. A Patent to Johannsen, U.S. Pat. No. 4,257,318 recognizes that a constant reference signal is necessary to assure stability in a control system, and Johannsen focuses on the use of a user set reference signal level to which are compared numerous air pressure representing signals, which air pressure representing signals are produced by sensors in various locations in air ducts in a house or building. The Johannsen approach selects the lowest such sensed air pressure representing signal and that signal is compared to the user set reference signal. Air volume flow rate is controlled based upon the signal resulting from the comparison. The Johannsen invention also provides for adjustable dead bands in the comparison circuitry to enhance stability. The problem with the Johannsen system is that, just as in the most basic large positive pressure schemes, the selected reference signal has no definite relationship to any relevant reference pressure, hence, the air volume flow rate can be unknowingly set to energy wasting levels which are higher than necessary to provide a healthy environment inside a house or building, over time. A Patent to Haines et al., U.S. Pat. No. 4,407,185 teaches the sensing of pressure in a plenum system and controlling air volume flow rates so that said pressure is typically maintained at a negative value with respect to outside air pressure. As a result outside air flows into the plenum. The reference signal is, however, derived from outside air pressure by a sensor which is exposed to wind, and thus the signal can be rapidly changed by wind, as has already been noted. It is added that while retaining a negative pressure in a plenum is an acceptable way to draw air into same, keeping a negative pressure in a house or building can lead to, for instance, flue gases can be trapped in a house or building by air being drawn down through a chimney. The results can be deadly. A Patent to Dean et al. teaches a system for use in hospitals. An air volume flow rate controlling signal is derived from the difference between air pressure signals derived by sensors located in a hospital room and in the hall outside the hospital room. An air volume flow rate is set, based upon the difference in said signals, which is sufficient to keep a positive or negative pressure in the room with respect to the pressure in the hall. While the pressure in the hall of a hospital will be relatively more constant than that outside the hospital, it will still change when doors are opened or closed etc. Again the reference pressure is variable.
It will be appreciated that the systems surveyed above provide air volume flow rate supplies which use reference signals which are simply set arbitrarily, or which are derived based upon references signals which are not relatively constant. As well, the basic approach is to provide air volume flow rates which are sufficient to keep a significant pressure differential in place. In either case the air volume flow rates provided will, over time, be in excess of what is actually needed to provide a "just adequate" ventilation, from a health maintenance perspective.
A need exists for a ventilation control system which identifies and utilizes a relatively constant pressure reference from which a signal can be derived, deviations from which signal can be used to control the air volume flow rate into, and out of, a house or building. Additionally, a need exists for a ventilation control system which does not typically maintain an excessive positive or negative pressure in a house or building, or part thereof, in excess of that which is just necessary to provide an adequate health maintaining, ventilation air volume flow rate into, and out of, the house or building, so that indoor air contaminant concentrations are kept below dangerous levels.